add -i (Perl version): color header to match the C version

add -i (Perl version): color header to match the C version

Both versions of `add -i` indent non-flat lists by five spaces. However
when using color the C version prints these spaces after the ANSI color
codes whereas the Perl version prints them before the color codes.
Change the Perl version to match the C version to allow for introducing
a test that verifies that both versions produce the exact same output.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>